The diminishing availability of high quality petroleum reserves encourages refiners to convert the greatest amount of low quality crudes to high quality light products such as gasoline. The majority of crudes which are currently available are very heavy, containing large amounts of low value residuum feeds which are unsuitable for catalytic cracking because of their tendency to foul or deactivate catalysts. These low value fractions are, however, suitable for use in producing delayed coker products.
Although the delayed coker unit is considered an economical and effective unit for making high quality products from low quality feeds, coker product yield and property distribution do depend on the type of feedstock available for coking. Thus, the refiner, to a certain degree, can control the coker products and the quality of coke by the choice of feedstock.
The delayed coking process is an established petroleum refinery process which is used on very heavy low value residuum feeds to obtain lower boiling cracked products. The lighter, lower boiling, components of the coking process can be processed catalytically, usually in the FCC unit, to form products of higher economic value. The solid coke product is used as is or is subjected to further processing.
Although the delayed coker unit is considered an economical and effective unit for making high quality products from low quality feeds, coker product yield and property distribution do depend on the type of feedstock available for coking. Thus, the refiner, to a certain degree, can control the coker products and the quality of coke by the choice of feedstock.
The main source of coker feedstocks include the bottoms of crude oil fractionators or vacuum columns, which are referred to as "short residuums" and "long residuums." The most common coker feedstocks are the short resids, or vacuum resids. These products have high metals and carbon contents. The hydrocarbon constituents in residuums are asphaltenes, resins, heterocycles and aromatics.
There are basically three different types of solid coker products which are different in value, appearance and properties. They are needle coke, sponge coke and shot coke. Needle coke is the highest quality of the three varieties. Needle coke, upon further treatment, has high conductivity and is used in electric arc steel production. It is low in sulfur and metals and is produced from some of the higher quality coker charge stocks which include more aromatic feedstocks such as slurry and decant oils from catalytic crackers and thermal cracking tars as opposed to the asphaltenes and resins.
Sponge coke, a lower quality coke, sometimes called "regular coke," is most often formed in refineries. Low quality refinery coker feedstocks having significant amounts of asphaltenes, heteroatoms and metals produce this lower quality coke. If the sulfur and metals content is low enough, sponge coke can be used for the manufacture of electrodes for the aluminum industry. If the sulfur and metals content is too high, then the coke can be used as fuel. The name "sponge coke" comes from its porous, sponge-like appearance.
Shot coke has been considered the lowest quality coke because it has the highest sulfur and metals content, the lowest electrical conductivity and is the most difficult to grind. The name shot coke comes from its shape which is similar to that of B-B sized balls. The shot coke has a tendency to agglomerate into larger masses, sometimes as much as a foot in diameter which can cause refinery equipment and processing problems. Shot coke is made from the lowest quality high resin-asphaltene feeds and makes a good high sulfur fuel source. It can also be used in cement kilns and steel manufacture.
Since recent refinery techniques in fluid catalytic cracking allow conversion of traditional coker feedstocks such as the high boiling hydrocarbons and residuum mixtures and heavy residuum feeds to lighter materials suitable for regular gasoline, high octane gasolines, distillates and fuel oils, refiners are finding it difficult to obtain the feedstocks necessary for making the solid coker products which are considered more valuable such as the needle coke and anode grade coke. The feedstocks available for coking are high resin-asphaltene feeds which cannot, yet, be processed effectively and efficiently in the FCC unit to produce gasoline, but which can be used to make shot coke.
In the delayed coking process, which is essentially a high severity thermal cracking, the heavy oil feedstock is heated rapidly in a fired heater or tubular furnace from which it flows directly to a large coking drum which is maintained under conditions at which coking occurs, generally with temperatures above about 450.degree. C. under a slight superatmospheric pressure. In the drum, the heated feed decomposes to form coke and volatile components which are removed from the top of the drum and passed to a fractionator. When the coke drum is full of solid coke, the feed is switched to another drum and the full drum is cooled and emptied of the coke product. Generally, at least two coking drums are used so that one drum is being charged while coke is being removed from the other.
When the coking drum is full of solid coke, the hydrocarbon vapors are purged from the drum with steam. The drum is then quenched with quench water to lower the temperature to about 200.degree. F. after which the water is drained. When the cooling step is complete, the drum is opened and the coke is removed by hydraulic mining or cutting with high velocity water jets.
A high speed, high impact water jet cuts the coke from the drum. A hole is bored in the coke from water jet nozzles located on a boring tool. Nozzles oriented horizontally on the head of a cutting tool cut the coke from the drum.
Even though the coking drum may appear to be completely cooled, occasionally, a problem arises which is referred to in the art as a "hot drum." This problem occurs when areas of the drum do not completely cool. This may be the result of a combination of morphologies of coke in the drum resulting in a nonuniform drum. That is, the drum may contain a combination of more than one type of solid coke product, i.e., needle coke, sponge coke and shot coke. BB-sized shot coke may cool faster than another coke, such as large shot coke masses or sponge coke. Usually, the lower quality coke is at the bottom of the drum and the higher quality coke is at the top of the drum.
The formation of zones in the coker drum which are impervious to cooling water can slow down the decoking process because these zones do not cool as quickly as the other, more pervious, zones of the drum. Such large agglomerations of coke can result in areas of high temperature or "hot spots." This condition is difficult to detect and may not be noticed by operating personnel. If the condition is detected, bottlenecking of the refinery occurs because the coking unit is out of operation for a longer length of time which is necessary to cool the drum before cutting the coke from the drum.
Alkali metal-containing materials which are used in hydrocarbon product finishing processes such as caustic extraction (such as treating in a UOP Merox unit), caustic scrubbing, mercapfining and hydrogen sulfide removal from liquid and gaseous refined hydrocarbon products are usually removed from the finished product by washing with water. The wash containing spent alkali is difficult to dispose. Refining with alkali is described in Dalchevsky et al, Petroleum Refining With Chemicals, pp. 137-175 (1958) and Bell, American Petroleum Refining, pp. 297-325 (1945) which are incorporated herein by reference in their entireties. The components of the spent alkali metal-containing materials not only contain the alkali metals of spent caustic soda and spent caustic potash which are themselves incompatible with the natural environment, but also contain process contaminants such as sulfur containing compounds and other waste, including some organic materials along with large quantities of water. Although the alkali metal-containing materials can be treated prior to disposal by incineration or oxidation in the liquid phase, their re-use in the refinery would be preferred.